My primary interest in the DK tank is Unholy, as you put it as an AE tanking spec. That, and of course the debuffs they do, in particular the one that works similar to Curse of Elements. (Can not for the life of me remember it's name at the moment Ebon Grasp or something). I think it will take some expiramentation, but will be as Ellay said a very viable option. In my opinion, it will also be fun and unique. Everyone's got Paladins, I have one of those already. And a Warrior, I want to try my hand at this one. The thing that conerns me really about the DK is the lack of a Shield, the fact that they seem to be based more on Dodge than Mitigation. It could make for some very spikey damage taken, and some severe healing issues.